The present invention relates generally to hearing aids, and more particularly, to circuits that more compactly, economically, and effectively present a modified and amplified sound for the wearer of a hearing aid. In its most basic form, a hearing aid is a device which receives a sound signal and furnishes a modified sound signal to the wearer of the hearing aid, so that the wearer may hear better.
Hearing impaired persons differ considerably in the degree and pattern of their hearing loss. This is reflected in their audiograms.
An audiogram is a chart of hearing threshold level (HTL) versus frequency. The HTL is measured on a logarithmic (decibel) scale and compares an individual's threshold of detection of a tone to that of normal hearing persons.
Audiograms can differ in level from near normal (0 dB HTL) to profound loss (greater than 100 dB HTL). They can vary in pattern from a flat audiogram (approximately equal HTL over the relevant frequency range) to a sharply falling audiogram (HTL increasing at more than 30 dB per octave of frequency increase) or to a rising audiogram (HTL decreasing with frequency). Within the category of sharply falling audiograms, the frequency at which the audiogram begins to fall can vary greatly between individuals.
Patterns of hearing loss can also differ considerably in the relationship between the subjectively experienced loudness and the input sound pressure level. Not only are there wide differences in the lowest level that the ear can perceive (the HTL), there are also wide differences in the highest level that the ear can tolerate (the loudness discomfort level or LDL) and differences in the rate of growth of loudness between these two extremes. In some cases, a much higher than normal HTL is combined with a lower than normal LDL, giving a much reduced dynamic range of usable sound levels.
Because of the large differences in degree and pattern of hearing loss, there is also a wide range of hearing aid characteristics required to optimally assist hearing impaired persons. The frequency response of the hearing aid must be selected for the individual hearing loss and may require high order filtering with selection of frequency bandwidth as well as general shape within that bandwidth. The gain of the hearing aid must be selected in accordance with the degree of the hearing loss.
The limiting level of the hearing aid must be selected in accordance with the LDL. A selection of the type of output limiting must be made between peak clipping, or output Automatic Gain Control ("AGC"). Output AGC is an automatic gain control system whose action is related to the output level of the hearing aid. This relation is substantially independent of the setting of the user operated volume control.
If the ear has a much reduced dynamic range, an input AGC system may be needed. Input AGC is an automatic gain control system whose action is related to the input level to the hearing aid. The relation between the action of the input AGC and the input level is also substantially independent of the setting of the user-operated volume control. In the case of an input AGC, a selection must be made of the compression threshold, (the input level at which AGC action begins), and the compression ratio, (the ratio of the decibel change in input to the decibel change in output level).
It is desirable that a single hearing aid be provided with a range of adjustability in these various characteristics. This is important not only because it allows a single hearing aid model to be used for many persons with differing hearing losses, but also because it allows the hearing aid to be readjusted if the initial selection of hearing aid characteristics was incorrect or if the user's hearing loss changes with time.
Because hearing aids are normally worn on the head, they must be small and usually operate with a small, single cell battery for extended periods of time. They must also provide good performance, be highly reliable, and be low in cost. The electronic circuits used to provide the various functions of a hearing aid must also have these same characteristics. Of course, such circuits will also be useful in applications other than hearing aids but having similar requirements.
Prior hearing aids have had limitations in meeting the needs described above. They have been limited in the degree of filtering provided to control the frequency response. When equipped with an input AGC system, they have not provided a well-defined compression threshold and a well-defined compression ratio. Also, they have not provided the needed degree of adjustability in either their frequency response characteristics or in their output level versus input level and AGC characteristics.
Prior electronic circuits also have had limitations in meeting the needs described above. Circuits that are small in size and that can operate with a low supply voltage of about 1.3 volts and that draw little supply current have not provided good performance and have not provided the desired adjustment characteristics. Prior electronic circuits that could meet the performance needs described above have required higher supply voltage and current, have required many components, have been large in size, or have not been in a form in which they could be adjusted by operation of a single control.
In many electronic circuit design applications, such as hearing aids, an input signal is provided. The input signal must then be modified by an appropriate filter. These modifications consist of attenuating the various frequency components of the input signal to differing degrees.
Thus, for example, a hearing aid wearer may have a more serious hearing impairment for sounds having a high frequency than for sounds having a low frequency. Accordingly, a hearing aid should amplify high frequency signals more strongly than the low frequency signals. An appropriate filter may then be used to allow high frequency signals to pass through it substantially unaltered and to substantially attenuate low frequency signals.
In the example described above, the filter is called a highpass filter, since high frequency signals are allowed to pass through the filter substantially unmodified. In addition, a lowpass filter allows only low frequency signals to pass through substantially unattenuated. Still others, known as bandpass filters, only allow signals having frequencies within a specified range to pass through the filter substantially unattenuated.
In designing a hearing aid, it is desirable to use a more effective and yet compact frequency response filter. Frequency responsive filters are commonly shown in the prior art. For highpass or lowpass filters, a predetermined frequency, known as the corner frequency or characteristic frequency of the filter, substantially divides those signals which are allowed to pass through the filter and those which are attenuated. One type of electronic frequency filter employed in a variety of applications is called a Butterworth filter. For example, in a lowpass Butterworth filter, signals having a frequency less than the corner frequency are allowed to pass through the filter substantially unimpaired. A signal with the frequency of the corner, however, is attenuated by approximately three decibles. Signals with a frequency in excess of the corner frequency are attenuated even more than three decibels.
Filters are further characterized by their number of poles. A pole is a complex frequency root of the denominator of the transfer function. The higher the number of poles, the higher will be the rate of attenuation of a Butterworth filter beyond the corner frequency. For example, a two pole highpass or lowpass filter will have a rate of attenuation of 12 decibels per octave and a four pole filter will have a rate of 24 decibels per octave. In many applications it is desirable to have a four pole filter.
Many applications require filters to be as compact and efficient as possible. For example, hearing aids often rest behind or inside the user's ear. Because of the resulting space limitations, all circuits within the hearing aid, including the frequency filter, should be small. Accordingly, the number of components in each circuit should be reduced.
Also, many applications require that the corner frequency of the filter be adjustable. For example, in hearing aids, users that require amplification of different frequencies may use the same hearing aid simply by adjusting the corner frequency of the filter. A user requiring amplification of signals with a frequency above 500 hertz and another second user, needing amplification of signals above 1000 hertz, may both use the same aid by adjusting the corner frequency of the filter, which determines the frequency response of the aid. Nonetheless, such an adjustable filter should still be as compact as possible.
In addition, the varying of the corner frequency of the filter should, if possible, use only a single control. This allows the adjustment of the corner frequency to be done more easily, as well as allowing the manufacture of the aid to be less expensive and providing a more reliable aid.
Furthermore, since a manually adjusted control element is often mounted in a location remote from the filter circuit, it is advantageous if the input signal does not pass through this control element, but rather that the control element varies a control signal which indirectly influences the corner frequency of the filter. Such an arrangement reduces difficulties that may be encountered with feedback, capacitive coupling, or the pickup of unwanted noise, which will affect the actual signal. Moreover, the control signal may be provided by a manually adjusted potentiometer or may be a signal originating with some other processing system within the aid itself. The control signal should be derived from a regulator that produces reference signals that accurately set the adjustment range of the corner frequency control.
In addition, to further decrease the size of the circuit, a large portion of the circuit should be formed on integrated circuits. In many cases, it is desirable to use semi-custom integrated circuits which contain large numbers of substantially identical circuit elements which may be interconnected as desired. Accordingly, the circuit should advantageously use greater numbers of integer multiples of such identical transistors in its design. Also, such integer multiples of substantially identical transistors may be used to increase the accuracy of the expected performance of the circuit.
Many circuit components used in hearing aids are formed on an integrated circuit chip. Transistors and amplifiers are readily available on such chips and usually do not take up inordinate amounts of space. Resistance elements, however, take up large amounts of space (or "real estate") on the chip. Moreover, the absolute value of a resistor formed on a chip is typically not easily kept within a close tolerance. Such typically wide tolerances would make the circuit performance less accurate. External discrete resistors interconnected with the chip may also be used, but such discrete components also use up much of the space available inside of an aid and also require additional connection points to the chip.
Other elements used in a circuit, such as capacitors, are difficult to fabricate on a chip. Of course, if discrete capacitors are used, they, like discrete resistors, take up space inside the aid, so that their number and size should be minimized. Also, the number of connection points to the chip should be minimized.
It is also helpful if the capacitors used have substantially equal values. In this way, the manufacturer need keep fewer items in inventory. Also, he may purchase larger quantities of the single type of capacitor (rather than smaller quantities of different types of capacitors), and thus possibly obtain the capacitors for a lower price. Consequently, the costs for both the manufacturer and consumer may be reduced. Moreover, the use of a single type of capacitor reduces the chance of "mixup," whereby an improper capacitor is used in the manufacture of the aid.
In addition, it is often desirable to have a common AC ground connection to one side of each capacitor. Such an arrangement tends to reduce the noise sensitivity of the circuit. Moreover, the circuit may then have fewer pad connections between capacitors and the integrated circuit, thereby reducing the cost and increasing the reliability of the circuit.
Of course, the filters should function properly with the voltage level supplied by a hearing aid battery, which is typically in the order of only one volt. Additionaly, the filter shall operate with a small current drain, so as to increase the operating life of the battery.
Many commonly available filters only provide a highpass or lowpass or bandpass transfer function, rather than the providing simultaneous highpass, lowpass, and bandpass outputs for a particular input signal. Simultaneous outputs are useful, for example, to split an input signal into highpass and lowpass channels.